Paper is sheet material containing interconnected small, discrete fibers. The fibers are usually formed into a sheet on a fine screen from a dilute water suspension or slurry. Paper typically is made from cellulose fibers, although occasionally synthetic fibers are used. Paper products made from untreated cellulose fibers lose their strength rapidly when they become wet, i.e., they have very little “wet strength”. Wet strength of ordinary paper is only about 5% of its dry strength. The wet strength of paper is defined as the resistance of the paper to rupture or disintegration when it is wetted with water. See U.S. Pat. No. 5,585,456. To overcome this disadvantage, various methods of treating paper products have been employed.
Paperboard is a thick paper based material that is used extensively for packaging applications. Wet strength retention can be a particularly desirable characteristic in paperboard that is used in carton, container, and packaging because of the prolonged exposure to moisture these materials may experience in these roles. However, paperboard often has poor wet strength properties, and conventional processes to impart wet strength may require additional process steps and add unwanted expensive. Therefore, there is a continuing need in the art for processes and compositions for imparting appropriate levels of wet strength to paper products, including paper and particularly paperboard.